1. Field of Invention
The present invention relates generally to semiconductor transducers, and more particularly to differential pressure transducers suitable for measuring differential pressure that employ piezoresistive sensors on a bossed diaphragm and stop members for limiting the deflection of the diaphragm upon the application of excessive force, and methods of making the same.
2. Description of Related Background Art
Semiconductor pressure transducers are employed in the measurement of pressure in numerous types of applications. Many pressure transducers employ a relatively thin diaphragm or deflecting member fabricated from semiconductor material such as silicon. The diaphragm is the thin portion of the transducer, which determines the pressure range of the device, which varies depending upon the thickness of the diaphragm. Typically upon the diaphragm is deposited or diffused a piezoresistive strain gage configuration, such as a bridge circuit, whereby the resistors associated with the bridge exhibit a change in resistance according to a deflection in the diaphragm as is well known. By monitoring the change in resistance, one obtains an output voltage indicative of the applied pressure or force.
One particular type of pressure transducer is a differential pressure transducer. Differential pressure transducers provide an output which is the difference between two pressures. For example, when a pressure P1 is applied to one face of the deflecting member and a pressure P2 is applied to the other face of the deflecting member, the resulting deflection will be determined by the difference in pressure, i.e., P1-P2. An example of differential piezoresistive bridge pressure transducers is illustrated in U.S. Pat. No. 6,272,928, entitled xe2x80x9cHermetically Sealed Absolute and Differential Pressure Transducer,xe2x80x9d assigned to the assignee herein.
Frequently, differential pressure transducers are subjected to a high xe2x80x9clinexe2x80x9d pressure. This high xe2x80x9clinexe2x80x9d pressure refers to the pressure which both sides of the sensor are subjected to while simultaneously measuring the difference in pressure from one side of the sensor to the other. Often the differential pressure is much smaller than the xe2x80x9clinexe2x80x9d pressure. In many instances, however, due to blockage in the line, the full xe2x80x9clinexe2x80x9d pressure applied to either side of the sensor separately thereby creates an enormous pressure difference across the sensor. In these instances, the deflection of the sensor in either direction must be restrained to avoid excessive strain on the diaphragm which causes it to fracture.
One known method of limiting the deflection of the sensor is to attach a deflection limiting member with a shallow cavity to the sensor, commonly referred to as a xe2x80x9cstop.xe2x80x9d The cavity depth is fixed to limit the deflection of the sensor a predetermined distance to thereby avoid excessive strain of the sensor. When a stop disposed on a sensor structure has no aperture to permit the passage of gas or air, the stop will only permit deflection until the sensor contacts the bottom of the cavity of the stop. In measuring an absolute pressure (unidirectional), it is apparent that this type of structure will limit the deflection of the sensor to insure that above a certain pressure, the sensor cannot deflect, thus preventing excessive strain on the sensor.
For bi-directional sensors, a stop without an aperture cannot be used. For the sensor to respond to pressure from either direction, the stops require apertures to allow the applied pressure to reach the deflecting sensor structure thereby causing it to deflect. Prior to the present invention, apertures have been located in the central portion of stop structures. To ensure adequate pressure application to the sensor, the central aperture in the stops should be as large as possible. To ensure the best stopping, however, the apertures in the stop should be as small as possible. When the apertures are disposed in the central portion of the stop structure, the region of the sensor where the most extensive deflection occurs does not entirely contact the bottom of the cavity of the stop where the aperture is located. Clearly, there are problems associated with stops having a central aperture, which the present invention seeks to avoid.
There still remains a need for a stop structure that provides improved stopping capabilities, while ensuring adequate pressure application to the sensor structure. There also remains a need for a stop structure that provides apertures that are large enough to insure adequate pressure application, while providing apertures that are small enough to insure the best xe2x80x9cstopping.xe2x80x9d There also remains a need for a semiconductor sensor that measures differential pressure across the sensor while being capable of withstanding high unidirectional pressure across the sensor.
Briefly described, a preferred embodiment of the present invention provides a semiconductor sensor for measuring differential pressure across the sensor diaphragm and operable in the presence of an excess unidirectional force applied to the sensor from either direction to stop said sensor diaphragm from further deflection during the presence of said excess force which tends to fracture said diaphragm. The transducer sensor includes a planar semiconductor diaphragm including a central active area on a top surface surrounded by a groove of a given width and depth which forms a central boss, the central area within the groove capable of deflection, and a peripheral non-active area surrounding the groove. The diaphragm has a relatively smooth bottom surface upon which piezoresistive sensors are formed that correspond in location on the bottom surface with the groove on opposing sides of the central boss of the top surface. A stop member is secured to the diaphragm at the peripheral area, and includes a first slotted aperture and a second slotted aperture in communication with the active area, the first and second slotted apertures extending generally along the length of the active area and which correspond in location with opposing sides of the central boss. The stop member includes a stop cavity located between the first and second slotted aperture, and the stop cavity overlies the central boss and is separated therefrom to enable the diaphragm to deflect when a force is applied and to enable the central boss to impinge on the surface of the stop cavity when an excessive force is applied. The first and second slotted apertures permit another force to be applied to the active region of the diaphragm in a direction opposite to the stopped direction.
In an alternate preferred embodiment of the invention, a stop member for a differential semiconductor sensor employing a bossed diaphragm includes a planar member having a first and a second slotted aperture, the first the second slotted apertures being substantially parallel and extending through the planar member. The first and the second slotted apertures are positioned so that when the stop member is disposed on a bossed diaphragm the first and the second slotted apertures are located along the outer edge of the active region of the diaphragm, and the slotted apertures are relatively as long as the active region. The stop member includes a stop cavity located between the slotted apertures, to allow a central boss of the diaphragm to impinge upon the surface of the stop cavity when an excessive force is applied to the diaphragm.
In an alternative preferred embodiment, the invention includes a double stop structure for a semiconductor pressure transducer employing a bossed diaphragm, including a first stop member that includes a plurality of cutout portions on the periphery thereof for accessing peripheral contact areas of a bossed diaphragm of a pressure transducer; a first cavity in a central portion of the first stop member for receiving the diaphragm of a pressure transducer; and two slotted apertures adjacent the first cavity and extending through the first stop member permitting access to the environment, and a second stop member including: a second cavity formed in a central portion of the second stop member for receiving the diaphragm, and two slotted apertures adjacent the second cavity and extending through said second stop member permitting access to the environment. The first stop and second stop member are operable under excessive force to prevent the diaphragm from excessive strain leading to fracture.
The present invention also provides a preferred method for manufacturing a differential pressure transducer that includes the steps of fabricating a bossed diaphragm including a first substrate composed of silicon, and a second substrate composed of silicon dioxide, the diaphragm including an central active region, and a peripheral non-active region separated by a groove which defines a central boss capable of deflecting, disposing a plurality of piezoresistive sensing elements on the active region, forming a plurality of contact areas on the diaphragm that extend from the sensing elements to the peripheral region, forming a first stop member including a cavity in the central portion thereof, a plurality of cutout portions of the periphery thereof, and two slotted apertures extending through the first stop member, and forming a second stop member including a cavity in the central portion thereof and two substantially parallel slotted apertures extending through the second stop member.